ifnet: Eliminate unnecessary synchronization of the interface index in the link layer address

Change 80e60e236d85 made the if_index global, and for the whole lifecycle
of an interface its index never changes, then there is no need to
synchronize the interface index in the link layer address when moving
interfaces from one vnet to another.

No functional change intended.

Reviewed by: glebius
Fixes: 80e60e236d85 ifnet: make if_index global
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50975